Biological and geological fractionations are often calculated differently, with
ID: 529271 • Letter: B
Question
Biological and geological fractionations are often calculated differently, with a positive Delta value in biology (such as^13C fractionation in photosynthesis) indicating a lower delta^13C value in your product than in your source, and a positive Delta value in geology (such as partitioning between quartz and magnetite) indicating a higher delta^13C value in your product than in your source. Explain why regardless of the approach taken, it is only approximately correct to calculate Delta as the difference between two measured delta^13C values. It is well known that stable isotopes fractionate according to the difference between masses. For example, the difference between^12C and^13C is one mass unit in 13:^12C and^13C are strongly fractionated in nature. Interestingly, the isotopes^40Ca and^48Ca do not fractionate as strongly in nature, yet the mass differential is larger (8 mass units in 48, so 1 in 6). What might explain this?Explanation / Answer
Isotopes are atoms of the same element that have different numbers of neutrons. Differences in the number of neutrons among the various isotopes of an element mean that the various isotopes have different masses. The superscript number to the left of the element designation is called the mass number and is the sum of the number of protons and neutrons in the isotope . For example, among the hydrogen isotopes, deuterium (denoted as D or 2H) has one neutron and one proton. The mass number is shown to the right of the element abbreviation, as in C-13 or C13 for carbon-13. So, C13 (carbon) decay at different rate. Now, Rayleigh equation is used for calculating the rate of partitioning of the isotropic atoms.
The isotopic literature abounds with different approximations of the Rayleigh equations, including the three equations below. These equations are so-named because the original equation was derived by Lord Rayleigh (pronounced "raylee") for the case of fractional distillation of mixed liquids. This is an exponential relation that describes the partitioning of isotopes between two reservoirs as one reservoir decreases in size. The equations can be used to describe an isotope fractionation process if: (1) material is continuously removed from a mixed system containing molecules of two or more isotopic species (e.g., water with 18O and 16O, or sulfate with 34S and 32S), (2) the fractionation accompanying the removal process at any instance is described by the fractionation factor a, and (3) a does not change during the process. Under these conditions, the evolution of the isotopic composition in the residual (reactant) material is described by:
REgardless of the source be it geological or biological, rate of partioning is same and this can be proved using above equation.
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